Seacoast roads under new threat as sea levels continue to rise

Durham
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A new study by the University of New Hampshire has found that coastal roads, some as far as two miles inland are at facing a new hazard that currently can't be seen by drivers - rising levels of groundwater caused by rising sea levels

There have been a number of studies that focused on possible damage to road infrastructure in coastal New Hampshire communities from surface water flooding due to rising sea levels. But little attention has been given to another problem that few people think about - the effects of climate change on groundwater.

In coastal New Hampshire, sea levels are forecast to rise between 3.9 to 6.6 feet (1.2 to 2.0 meters) by 2100. It may be hard to wrap your mind around the concept, but groundwater levels are expected to rise along with rising sea levels, and without drastic road improvements now, motorists can expect segments of these roadways to deteriorate more quickly and the cost of maintenance will be greater.

The study, published in the online journal, Transportation Research Record, found that as sea levels rise, a corresponding rise in groundwater levels will occur. This is because groundwater levels are higher than sea levels and this drives groundwater discharge to the ocean.

A rise in sea-level will affect ground-water flow in coastal aquifers (1). An increase in the elevation of the water table (dashed–blue line) may result in basement flooding and compromise septic systems (2). A rise in sea level may also result in an upward and landward shift in the position of the freshwater-saltwater interface (3). Where streams are present, an increase in the water-table elevation also may increase ground-water discharge to streams (4).

USGS

As sea levels rise, this forces groundwater levels to slowly rise in order to maintain equilibrium, changing the elevation of the freshwater-saltwater interface. This natural action will bring groundwater closer to the pavement's base layers and they need to be kept dry and well-drained to protect the integrity of the road.

Jo Daniel, professor of civil and environmental engineering, director of UNH's Center for Infrastructure Resilience to Climate, and co-author on the study pointed out that water is a highway's worst enemy.

"If the soil and substrate under the pavement get wet, then the strength that we had counted on to carry the traffic isn't there anymore. So the pavement develops ruts and cracks, allowing more water to get into the underlying layers which makes the situation worse and closing roads for long periods of time to dry out impacting both commuters and tourists," Daniel said, according to Science News Online.

Since 2005, New Hampshire has experienced an increase in the frequency and severity of flooding: Heavy rains, high rivers, disaster spending and news coverage of flooding all set new records during this decade.

University of New Hampshire

Jayne Knott, a civil engineering doctoral candidate in UNH's College of Engineering and Physical Sciences and lead author of the study spoke with Phys.Org, saying, "We found that the effects of surface water flooding on roads occur within a mile of the coast, and groundwater rise effects can occur more than twice that, sometimes all the way to Pease Tradeport."

The research team studied both major highways, which are built with thicker cross-sections of materials to withstand heavier traffic, as well as roads in small towns along the coast. Roads in some small towns are often little more than a layer of pavement over a shallow layer of gravel.

N.H. Department of Transportation (NHDOT) road cross-section data with current and projected groundwater levels was also examined to correlate with the cross-sectional data on the different highways and roads in the study. The researchers found that all the evaluation sites experienced service life reduction, the magnitude, and timing of which depended on the current depth to groundwater.